In the manufacture of paper from wood, the wood is first reduced to an intermediate stage in which the wood fibers are separated from their natural environment and transformed into a viscous liquid suspension known as a pulp. There are several classes of techniques which are known, and in general commercial use, for the production of pulp from various types of wood. The simplest in concept of these techniques is the so-called refiner mechanical pulping (RMP) method, in which the input wood is simply ground or abraded in water through a mechanical milling operation until the fibers are of a defined desired state of freeness from each other. Other pulping methodologies include thermo-mechanical pulping (TMP), chemical treatment with thermo-mechanical pulping (CTMP), chemi-mechanical pulping (CMP) and the so-called kraft or sulfate process for pulping wood. In all of these processes for creating pulps from wood, the concept is to separate the wood fibers to a desired level of freeness from the complex matrix in which they are embedded in the native wood.
Of the constituents of wood as it exists in its native state, the cellulose polymers are the predominate molecule which is desired for retention in the pulp for paper production. The second most abundant polymer to cellulose in the native wood, which is the least desirable component in the pulp, is known as lignin. Lignin is a complex macromolecule of aromatic units with several different types of interunit linkages. In the native wood, lignin physically protects the cellulose polysaccharides in complexes known as lignocellulosics, and those lignocellulosics must be disrupted for there to be marked enzyme accessibility to the polysaccharides, or to separate lignin from the matrix of the wood fibers.
It has been suggested that biological systems can be utilized to assist in the pulping of wood. A desirable biological system would be one which is intended to liberate cellulose fibers from the lignin matrix by taking advantage of the natural abilities of a biological organism. Research in this area has focused on a type of fungi referred to as white-rot wood decay fungi. These fungi are referred to as white-rot, since the characteristic appearance of wood infected by these fungi is a pale color, which color is the result of the depletion of lignin in the wood, the lignin having been degraded or modified by the fungi. Since the fungi appear to preferentially degrade or modify lignin, they make a logical choice for fungi to be utilized in biological treatments to pulp wood, referred to as biopulping.
Several reports have been made of attempts to create biopulping systems using white-rot fungi on a variety of wood fibers. Previous research has concentrated on a single, or relatively few, species of fungi. The most commonly utilized fungi in such prior systems is the white-rot fungi Phanerochaete chrysosporium, also referred to as Sporotrichum pulverulentum. Other fungi which have been previously used in such procedures include fungi of the genera Polyporus and Phlebia. The prior art is generally cognizant of the fact that attempts have been made to use biological organisms, such as white-rot fungi, as part of a process of treating wood, in combination with a step of either mechanical or thermal mechanical pulping of cellulose fiber.
The use of white rot fungi for the biological delignification of wood was studied as early as the 1950s at the West Virginia Pulp and Paper Company (now Westvaco) (Lawson and Still, C. N. (1957) Tappi J., 40, 56A–80A). In the 1970s Eriksson and coworkers at STFI (Swedish Forest Product Laboratory) demonstrated that fungal treatment could result in significant energy savings for mechanical pulping (U.S. Pat. No. 3,962,033 for an invention by Eriksson et al. (1976); (Ander and Eriksson, K. E., (1975); Svensk Papperstidning, 18, 641) (Eriksson and Vallander, K. E. (1982) Svensk Paperstidning, 85(6), R33–R38). Two sequential biopulping consortia comprised of the USDA Forest Service, Forest Products Laboratory in Madison, Wis. (hereinafter, “FPL”), the Universities of Wisconsin and Minnesota, and 22 pulp and paper and allied companies demonstrated the techno-economic feasibility of biopulping in connection with mechanical refining (Akhtar et al., (1992a), Tappi J., 75(2), 105–109); (Akhtar et al., (1992b) Biotechnology in the pulp and paper industry, (Kuwahara, M. and Shimada, M. eds.) Tokyo, UNI Publishers Company Ltd., p. 545); (Akhtar et al., (1993) Holzorschung, 47(1), 36–40); (Blanchette, R., (1984) Applied & Environmental Microbiology, 48(3), 647–653); (Blanchette et al., (1988) Biomass, 15, 93–101); Leatham et al.(1989) Biotechnology in the Pulp and Paper Industry, 4th International Symposium, Raleigh, N.C., May 16–19); (Leatham et al., (1990a), Tappi J., 73(3), 249–255); Leatham et al., (1990b), Tappi J., 73(5), 197–200), (Myers et al., (1988), Tappi J., 71(5), 105–108); (Pearce, N. H., et aL) screened 204 isolates of wood decay fungi in bench scale trials for their performance in biomechanical pulping of eucalyptus chips. (Proccedings 49th Appita Annual General Conference, Hobart, Tasmania, Australia, 2–7 Apr. 1995, 347–351) Refining energy savings of 40%–50% were obtained with some selected fungi. No strength improvements were reported. Additional developments in biomechanical pulping were described in: U.S. Pat. No. 5,055,159 for an invention by Blanchette, et al. (1991); U.S. Pat. No. 5,460,697 for an invention by Akhtar et aL (1995); U.S. application published as WO 9605362 on Feb. 1, 1996.
Unfortunately, biomechanical processes have only gained limited commercial acceptance, and have not been widely utilized. One of the difficulties has been that most of the prior techniques for utilizing biological techniques for the pulping of paper have resulted in paper which has had only marginal strength increase or is weaker than papers made by more conventional processes.
In fact, while a certain amount is known about the interaction of lignin and cellulose in wood fibers, because of the extreme complexity of the relationships, and the variation in the enzymes produced by varieties of the white-rot fungi, it is not readily possible to predict from the action of a given fungus on a given type of wood whether or not the paper made from wood partially digested with such fungus will have desirable qualities or not. The selection of white-rot fungi for biopulping applications on the basis of selective lignin degradation may seem a rational one, but it has proven to be a poor predictor of the quality of the resultant paper. The exact relationship between the degradation of lignin, and the resulting desirable qualities of paper produced at the end of the pulping process, are not at all clear. Accordingly, given present standards of technology and the present understanding of the complex interaction of lignin and cellulose, it is only possible to determine empirically the quality of paper produced through a given biological pulping process and the amount of any energy savings achieved through such a process.
For reasons set forth above, most of the fungi screened for the biomechanical pulping of one type of wood do not necessarily work well in the biomechanical pulping of another type of wood. All the biomechancial pulping references described above are directed to the biopulping and processing of wood species other than eucalyptus, a very common wood species in many parts of the world and potentially valuable source of pulp for papermaking or other processes. What is needed is a method of processing eucalyptus wood which takes advantage of the cost savings of mechanical pulping techniques without a loss of end product quality one often experiences when using mechanical pulping.